1. Field of the Invention
This invention relates generally to a tensioner for tensioning a belt of a belt drive system. More particularly, it relates to a tensioner having a linear pulley movement. Specifically, this invention relates to a linear movement tensioner having a mechanical damping mechanism.
2. Description of the Prior Art
Power transmission belt tensioners used in conjunction with accessory belt drive systems for internal combustion engines are known with pulley movements that are both arcuate and linear. Certain applications benefit from linear movement tensioners (i.e., those where the idler pulley contacting the power transmission belt moves in a substantially straight path). An example of a linear movement tensioner can be found in U.S. Pat. No. 4,634,408 to Foster. The ""408 Patent discloses the construction of a relatively complex tensioner that incorporates a plurality of springs about a hydraulic damping mechanism. Further, the carrier, upon which the tensioning pulley is mounted, engages the track, that defines the carriers two degrees of freedom of movement, with a ball and track bearing structure. The inclusion of hydraulics creates the concern of leaking hydraulic fluid. The hydraulic mechanism disclosed is apparently not compact. Thus, the tensioner appears to be relatively large. The relatively large number of parts adds to cost and complexity of construction. Further, there is no provision for compensating for the parasitic torque across the carrier that supports the idler pulley, that tends to twist that carrier, other than the effectiveness of the ball and track bearing structure.
Accordingly, there is a continuing need for a linear movement tensioner that is simple, compact, durable and devoid of hydraulics.
The present invention has as an object the provision of a linear movement tensioner that is simple and compact.
The present invention has as a further object the provision of a linear movement tensioner with a mechanical damping mechanism having asymmetrical damping properties.
The present invention has as a further object the provision of a linear movement tensioner with a mechanical damping mechanism having parasitic torque compensating properties to enhance tensioner durability and operation.
To achieve the foregoing and other objects in accordance with the purpose of the present invention, as embodied and broadly described herein, an accessory drive system is disclosed herein. The invention is an improved tensioner for tensioning a power transmission belt. The tensioner is of the type having a track, a carrier mounted in sliding relation with the track with two degrees freedom of movement, a pulley rotatably mounted upon the carrier and for engaging the power transmission belt, a spring biasing the carrier in longitudinal relation to the track, and a damping mechanism that modifies the biasing of the spring based upon movement of the carrier in relation to the track. It is improved by the damping mechanism having a shoe placed in asymmetrical damping relation to the carrier and the track. Further, the shoe includes a first friction bearing surface in mating relationship with a second friction bearing surface of the track.